Wafer level alignment structures using subwavelength grating polarizers

ABSTRACT

In one embodiment, a wafer alignment system, comprises a radiation source to generate radiation, a radiation directing assembly to direct at least a portion of the radiation onto a surface of a wafer, the radiation having a polarization state, an optical analyzer to collect at least a portion of the radiation reflected from the wafer, the wafer including at least a first region having a first grating pattern oriented in a first direction and at least a second region having a second grating pattern oriented in a second direction, different from the first direction.

BACKGROUND

The subject matter described herein relates generally to semiconductorprocessing, and more particularly to wafer level alignment structuresusing subwavelength grating polarizers.

During semiconductor device fabrication processing numerous (e.g.,hundreds of) integrated circuit chips or die, each having millions ofelectronic devices, may be formed on a single semiconductor wafer. Thewafer may be a thin disk or round slice of semiconductor material (e.g.,silicon), such as to provide a semiconductor (e.g., silicon, germanium,or a combination thereof) or semiconductor on insulator (SOI) substrateon or in which to form the electronic devices.

Similarly, targets (e.g., wafer alignment optical targets) may be formedon or in layers of the wafer to accurately align the wafers forprocessing to form the devices (and to form the targets). A large numberof semiconductor device fabrication processing tools perform some levelof basic alignment on semiconductor wafers prior to processing them.Alignment may be used to determine a radial, rotational, twodimensional, three dimensional and/or a coordinate system based positionor location on the wafer. The fabrication process may include a devicefabrication process to form circuit features or structures of electronicdevices (e.g., to form gate structures, diffusion regions, sources,drains, dielectric layers, gate spacers, shallow trench isolation (STI),integrated circuits, conductive interconnects, metal or alloy features,metal or alloy traces, metal or alloy contacts, and the like oftransistors, resistors, capacitors and the like). The device fabricationprocess may include forming layers of circuitry or circuit features inor on layers of the wafer. The fabrication process may end with a dicingprocess to separate or “saw” the wafer into distinct chips or die. Toaccurately align the wafer to form the electronic device circuitry atproper locations (e.g., regions or portions of the wafer surface and/orlayers below the surface), a wafer scanning (e.g., a wafer inspection oralignment) process may be used during the device fabrication process,and/or dicing process. The inspection or alignment process typicallyincludes locating one or more optical pattern recognition targets toindex the wafer.

Wafer alignment is typically accomplished by using pattern recognitionsoftware to locate specific targets, structures, or features on wafersand then correcting for wafer position relative to the tool's waferstage. Examples of such equipment include scanner, critical dimensionscanning electron microscopy (CDSEM), litho registration, defectinspection, and film thickness tools. In general, the alignment systemsused on these tools are based on optical microscopes (e.g., alignment orinspection microscopes). In order for the pattern recognition to besuccessful, the target structures need to have sufficiently high opticalcontrast relative to their surrounding background. In some processtechnologies, targets are created by lithographically patterning large,isolated solid features (few tens of microns in size) on wafers. Thesepattern recognition targets are constructed from large patterns arrangedin some unique shape. For some fabrication process technologies, thelarge patterns may have defect issues due to line-width and/or criticaldimension (CD) control requirements. For example, in some fabricationprocess technologies, linewidth and CD control requirements dictate verytight control of solid feature size and pattern density in order toenable the patterning process and avoid defect issues. For example, useof large, non-design rule-compliant solid features in some fabricationtechnologies would result in polish (e.g., chemical mechanical polishing(CMP) to planarize a surface of the wafer) dishing, film delamination,and/or defect generation, depending on the layer. These structures tendto be incompatible with such rules given that they need to be fairlylarge in order to be observable under an alignment microscope.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed embodiments will be better understood from a reading ofthe following detailed description, taken in conjunction with theaccompanying Figures in the drawings in which:

FIG. 1 is a schematic illustration of a wafer of alignment system,according to embodiments.

FIGS. 2A and 2B are schematic top perspective view of examples ofpattern recognition targets in regions of a wafer that may be used forwafer alignment.

FIG. 3 is a schematic illustration of a plot of reflectivity as afunction of grating pitch for each polarization of reflected radiation.

FIGS. 4A-4C are schematic illustrations of wafer-level alignmentstructures according to embodiments.

FIG. 5 is a flowchart illustrating a method detecting an alignment of awafer, according to embodiments.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of a wafer of alignment system 100,according to embodiments. Referring to FIG. 1, in an embodiment a system100 comprises an optical system 130 and an analysis system 160. Theoptical system 130 comprises a radiation source 132, a polarizer 134,and a collector 136. In some embodiments, radiation source 132 emitsradiation having a wavelength that measures between 350 nanometers (nm)and 800 nm. In alternate embodiments radiation source 132 may generateincoherent light.

Radiation emitted from radiation source 132 is transmitted to polarizer134, which polarizes radiation from radiation source 132 into either alinear polarization or an elliptical polarization state. In someembodiments polarizer 134 may be implemented as a polarizing beamsplitter, such as a Wollaston prism. In other embodiments, polarizer 134may be implemented as a thin film polarizer, a birefringent polarizer,or the like. Optical system 130 may include additional opticalcomponents such as, e.g., lenses, filters, mirrors, collimators, or thelike. Radiation source 132, polarizer 134, and any additional opticalcomponents associated with directing radiation toward wafer 120 may beconsidered a radiation directing assembly.

At least a portion of the radiation reflected from the surface of wafer120 is collected by collector 136. In some embodiments, collector 136may be implemented as an optical microscope.

The analysis system 160 comprises a memory module 162, an input/outputmodule 164 and a processor 166 and may be integrated within system 100or may be a stand-alone computer system coupled to the system 100.

FIGS. 2A and 2B are schematic top perspective view of examples ofpattern recognition targets 210A, 210B in regions of a wafer that may beused for wafer alignment. Pattern recognition targets 210A, 210B may becollectively referred to herein by reference 210. The targets 210 may beconstructed from large patterns or structures of segmented (i.e.grating) features arranged in some unique shape on or in wafer 120(e.g., on a surface or topmost surface of wafer 120). The targets 210include at least a first region 210 having a first grating patternoriented in a first direction and at least a second region 215 having asecond grating pattern oriented in a second direction, different fromthe first direction. Space in the embodiment depicted in FIGS. 2A andFIG. 2B, the first direction and the second direction are substantiallyperpendicular to one another.

For example, FIG. 2A shows target 200A as a large structure having afirst region 210A having a plurality of grating lines oriented in ahorizontal position, and a second region 215A having a plurality ofgrating lines oriented in a substantially vertical position. Similarly,FIG. 2B shows target 200B as a large structure having a first region210B having a plurality of grating lines oriented in a horizontalposition, and a second region 215B having a plurality of grating linesoriented in a substantially vertical position. Second region 215B formsa checkerboard pattern within first region 210B.

Each region 210, 215 may be larger than the minimum resolution size(e.g., are greater than a best resolution limit possible) of a waferalignment system inspection microscope (e.g., each of those features maybe larger than a minimum size pixel the optical inspection microscopecan focus on, thus the microscope can “see”, focus on, or identify thosefeatures and their location). In some cases, each region 210, 215 may belarger or equal to a 100 microns squared (μm²) area. Thus, themicroscope is able to clearly see, form an image of, or identify eachfeatures shape and edges.

According to embodiments, pattern recognition targets including one ormore regions 210, 215 of gratings (e.g., instead of a flat surface of asingle material, a region of grating has grating features such as gratesof a single material, another different material between the grates,and/or spaces between the grates) can be used with or instead of targetshaving only single material solid features. In some embodiments, regions210 may be distinguished from regions 215 by illuminating the surface ofthe wafer 120 with polarized light and distinguishing between reflectionpatterns generated by regions 210 and 215, respectively.

When light shines onto a reflective grating surface, as shown in FIGS.2A and 2B, light reflects off of the grating at discrete angles definedby

θ_(n)=sin⁻¹(nλ/Λ−sinθ_(i))   (1)

where θ_(i) is the incident angle, Λ is the pitch of the grating, λ isthe wavelength of the light, n=0 is referred to as the zeroth order orreflected light, while n=±1, ±2, . . . is referred to as the diffractedorders. When the grating pitch is sufficiently smaller than thewavelength (Λ<˜350 nm for visible light), then little if any of theincident light is diffracted. In this regime, the zeroth order/reflectedlight accounts for nearly all of the light leaving the grating.

The grating may be modeled using effective media theory as a uniformmedia with different refractive indices for light polarized parallel vs.perpendicular to the grating grooves. The “effective media” exhibits thefollowing refractive indices for the parallel and perpendicularpolarizations:

n _(parallel) =[n ₁ ² q+n ₂ ²(1−q)]^(1/2)   (2)

n _(perpendicular)=[(q/n ₁ ²)+(1−q)/n ₂ ²]^(−1/2)   (3)

where q is the duty cycle of the grating. As a result, light reflectingoff of the grating experiences a polarization-dependent phase shift.Using (2) and (3), the reflectivity (i.e. ratio of reflected light toincident light) of the grating for each polarization may be expressed inthe form

R _(parallel) =A+B* cos(4πn _(parallel) d/λ)   (4)

R _(perpendicular) =A+B* cos(4πn _(perpendicular) d/λ)   (5)

where A and B are constants (=function of n1, n2, n3), and d is theheight of the grating grooves.

From equations (4) and (5), it is apparent that the reflectivity variessinusoidally with refractive index and that furthermore, since eachpolarization has a different refractive index, the reflectivity of eachpolarization is different. By choosing the grating parameters properly(n1, n2, n3, λ, q, Λ), reflectivity can be increased (or maximized) forone of the incident polarizations (i.e. 4πn_(parallel)d/λ=even multipleof π, while minimizing it for the other (i.e. 4πn_(perpendicular)d/π=odd multiple of π. In this manner, the grating structure behaves asan optical polarizer.

FIG. 3 is a schematic illustration of a plot of reflectivity as afunction of grating pitch for each polarization of reflected radiation.In this example, for q=0.35, the reflectivities for the parallel andperpendicular polarizations are 5.9% and 32.9%, respectively. Theeffective media approach presented here is only an approximation. Arigorous analysis of the polarization of the reflected light isconsiderably more complicated. Nevertheless, it serves to illustrate howa grating can function as a polarizer.

Consequently, wafer alignment may be accomplished by using patternrecognition software to locate (e.g., to “see”, focus on, find, and/oridentify) specific targets or target features having gratings in or onwafers and then correcting for wafer position relative to the tool'swafer stage. Such targets, features, and or gratings may be described asin, on, of, or for a wafer. For instance, a wafer processing tool (e.g.,a semiconductor device fabrication processing tool) may include a waferalignment system having an inspection microscope to inspect a wafer“surface” (e.g., the topmost surface as well as and/or layers below thetopmost surface) by radiating or exposing the surface to light (e.g.,incident light through a numerical aperture of the system or microscopeand onto the surface) and collecting, receiving and/or measuring thelight reflected and diffracted back to the system or microscope (e.g.,back through or within the numerical aperture). The light may be whitelight, have a specific wavelength spectrum or color, and/or including orhave a “centroid wavelength”. The targets and/or features may beidentified and/or located by the brightness, darkness and/or contrastbetween brightness and darkness of light reflected back from regions ofor areas of one or more layers of gratings. The alignment system mayalign the tool with a wafer, and/or a portion, region, and/or field ofthe wafer sufficiently for the tool to be able to accurately andsuccessfully perform device fabrication processes at the portion,region, and/or field of the wafer.

Also, the appearance of the alignment targets described herein may becontrolled primarily by the pitch and positioning of the two gratingsrelative to each other. Although the individual grating features (e.g.,dimensions) forming the gratings may be below the resolution limit ofthe alignment microscopes, contrast may be achieved by controlling thediffractive and polarizing properties of the grating. In other words, aregion of grating may be detectable by an inspection microscope, eventhough the grates, material and/or space between the grates (e.g.,grates or features of a grating) of the grating have physical dimensions(e.g., width) that are smaller or below the minimum resolution size ofan inspection microscope (e.g., smaller than the minimum size pixel,solid feature, or grating feature the microscope can form an image of,such as on a screen). The detection may be possible because the regionhas a uniform or consistent “dark”, “bright” or “gray” brightnessdetected by the microscope as compared to adjacent regions, which mayhave a different brightness and/or which may define an edge between theregions.

Thus, according to embodiments alignment structures may be designed totake advantage of the polarizing nature of reflection gratings to createoptical contrast on the surface of the wafer. Reflection gratings may bedesigned with different reflectivities for different polarizationorientations and patterned onto wafers using conventional lithography.For example, alignment gratings may be included in the scribelines ofproduction reticles and be patterned onto wafers during the variouslithography operations in the process flow. In some embodiments, tworegions of gratings may be formed on a wafer: a “feature region” and a“background region”. The gratings forming the feature and backgroundregions may be arranged such that the lines forming the gratings in eachregion are rotated by 90 degrees to each other, as shown in FIGS. 2A and2B. In this example, the lines forming the feature region are vertical,while the lines forming the background region are horizontal. Whenilluminated by linearly polarized light, with the electric field vectororiented parallel or nearly parallel to one set of lines, one of theregions would appear “bright” (i.e. very reflective) while the otherregion would appear “dark” (i.e. less reflective).

In some embodiments, the background and feature regions can be patternedonto a wafer in a single reticle exposure, allowing the gratingalignment structure to be used at subsequent processing steps. If needed(for example to meet pattern density design rules at multiple processlayers), the same pattern can be replicated at multiple lithographyoperations. Additionally, the grating patterns created at differentlithography layers may be stacked on top of each other to enhance thepolarization effect.

FIGS. 4A-4C are a schematic illustrations of wafer-level alignmentstructures according to embodiments. Referring to FIG. 4A, the lithoregistration metrology structures may be designed using a variety ofdummification strategies. The dummification pattern consists of nestedlines and spaces, which may be drawn on the reticle. The top and bottomquadrants of the dummification consist of pattern vertical lines. Theleft and right quadrants consist of horizontal lines. The dummificationis intended to satisfy pattern density design rules without interferingwith registration measurement. For example, the dummification patternshown in FIG. 4A may be lithographically patterned on a wafer. Thispattern consists of nested lines and spaces, oriented eitherhorizontally or vertically, with a pitch of 320 nm. Registrationmeasurement features may be patterned on top of the dummificationpattern at subsequent lithography steps, forming the structure shown inFIG. 4B. When the dummification pattern on a actual wafer is illuminatedusing polarized radiation, for example under a registration tool'salignment microscope, the portions of the dummification patterncomprised of vertical lines appeared dark, while the portions comprisedof horizontal lines appeared light, as shown in FIG. 4C.

FIG. 5 is a flowchart illustrating a method detecting an alignment of awafer, according to embodiments. Referring to FIG. 5, at operations 510,a first grating pattern is formed on the surface of the wafer. Forexample, the first grating pattern may be formed using a firstlithographic etching process. The first grating pattern includes aplurality of gratings oriented in a first direction. At operation 515 asecond grating pattern is formed on the surface of the wafer. Forexample, the second grating pattern may be formed using a secondlithographic etching process. The second grating pattern includes aplurality of gratings oriented in a second direction. In someembodiments the first direction and the second direction aresubstantially orthogonal. In other embodiments, the first direction andthe second direction need not be perfectly orthogonal; rather, minordeviations from orthogonal may be implemented. In some embodiments, thefirst grating pattern has a pitch that is less than the wavelength ofthe radiation generated by radiation source 132. Similarly, in someembodiments, the second grating pattern has a pitch that is less thanthe wavelength of the radiation generated by radiation source 132.Additionally, in some embodiments, the first and second grating patternsmay be formed in the same lithography process.

At operation 520, radiation is directed onto the surface of the wafer.For example, with reference to FIG. 1, radiation source 132 may generateradiation, and polarizer 134 may polarize radiation from radiationsource 132, which may directed onto the surface of wafer 120. Atoperation 525, radiation reflected from the surface of wafer 120 iscollected. For example, radiation may be collected by radiationcollector 136. At operation 530, a wafer alignment may be determinedusing the radiation reflected from the surface which was collected byradiation collector 136. Space for example, analysis system 160 maygenerate electrical signals corresponding to the intensity of thecollected radiation. Because the incident radiation is polarized,radiation reflected from the first grating pattern will exhibit a firstintensity level, and radiation reflected from the second grating patternwill exhibit a second intensity level, different from the firstintensity level. Therefore, the grating patterns will be visible asdifferent intensity levels of reflected radiation. The border linesbetween the first region and the second region may be used to demarcateone or more locations on the surface of the wafer. These locations maybe compared to data mapping the surface of the wafer in order todetermine an alignment of the wafer.

In the description and claims, the terms coupled and connected, alongwith their derivatives, may be used. In particular embodiments,connected may be used to indicate that two or more elements are indirect physical or electrical contact with each other. Coupled may meanthat two or more elements are in direct physical or electrical contact.However, coupled may also mean that two or more elements may not be indirect contact with each other, but yet may still cooperate or interactwith each other.

Reference in the specification to “one embodiment” “some embodiments” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least an implementation. The appearances of the phrase “in oneembodiment” in various places in the specification may or may not be allreferring to the same embodiment.

Although embodiments have been described in language specific tostructural features and/or methodological acts, it is to be understoodthat claimed subject matter may not be limited to the specific featuresor acts described. Rather, the specific features and acts are disclosedas sample forms of implementing the claimed subject matter.

1. A wafer alignment system, comprising: a radiation source to generateradiation; a radiation directing assembly to direct at least a portionof the radiation onto a surface of a wafer, the radiation having apolarization state; an optical analyzer to collect at least a portion ofthe radiation reflected from the wafer; the wafer including at least afirst region having a first grating pattern oriented in a firstdirection and at least a second region having a second grating patternoriented in a second direction, different from the first direction. 2.The system of claim 1, wherein: the radiation source generates radiationhaving a wavelength; the first grating pattern has a pitch that is lessthan the wavelength; and the second grating pattern has a pitch that isless than the wavelength.
 3. The system of claim 1, wherein theradiation directing assembly comprises at least one polarizer topolarize the radiation to a linear polarization state or an ellipticalpolarization state.
 4. The system of claim 1, wherein the first gratingpattern and the second grating pattern are formed in the same layer ofmaterial of the wafer.
 5. The system of claim 1, wherein the firstgrating pattern and the second grating pattern are formed in differentlayers of material of the wafer.
 6. The system of claim 1, wherein thefirst direction is substantially perpendicular to the second direction.7. The system of claim 1, wherein the wafer comprises a plurality ofregions having a first grating pattern oriented in a first direction anda plurality of regions having a second grating pattern oriented in asecond direction, substantially perpendicular to the first direction. 8.The system of claim 1, wherein radiation reflected from the firstgrating pattern has a first polarization orientation and radiationreflected from the second grating pattern has a second polarizationorientation.
 9. The system of claim 8, wherein the optical analyzerdistinguishes between radiation reflected from the first grating patternand radiation reflected from the second grating pattern.
 10. A method todetect an alignment of a wafer, comprising: forming a first gratingpattern in a region of the wafer during a fabrication process, the firstpattern including a plurality of gratings oriented in a first direction;forming a second grating pattern in a region of the wafer during afabrication process, the second pattern including a plurality ofgratings oriented in a second direction, different from the firstdirection; directing radiation onto a surface of a wafer, the radiationhaving a polarization state; collecting at least a portion of theradiation reflected from the wafer; and detecting an orientation of thewafer using a characteristic of radiation reflected from the firstregion and the second region.
 11. The method of claim 9, wherein:forming a first grating pattern in a region of the wafer during afabrication process comprises performing a first lithographic etchingprocess; and forming a second grating pattern in a region of the waferduring a fabrication process comprises performing a second lithographicetching process, different from the first process.
 12. The method ofclaim 9, wherein directing radiation onto a surface of a wafercomprises: generating radiation having a wavelength; and polarizing theradiation to have an elliptical polarization state or a linearpolarization state.
 13. The method of claim 11 wherein: the firstgrating pattern has a pitch that is less than the wavelength; and thesecond grating pattern has a pitch that is less than the wavelength. 14.The method of claim 12, wherein radiation reflected from the firstgrating pattern has a first intensity and radiation reflected from thesecond grating pattern has a second intensity, the difference in thoseintensities providing the optical contrast to differentiate the twograting patterns.
 15. The method of claim 13, wherein detecting anorientation of the wafer using a characteristic of radiation reflectedfrom the first region and the second region comprises distinguishingbetween the first intensity and the second intensity.